chore(ci): add weekly scheduled CI run against main

[w-martin]

Catches toolchain/dependency drift on main within 7 days rather than at the next PR (the failure mode in #20).

Summary

• Adds a weekly Monday 06:00 UTC schedule: trigger to the on: block
• lint-and-test runs automatically on the schedule — no job-level changes needed
• Tag-gated jobs (build_wheels, build_sdist, publish) are unaffected — they only fire on v* tags

Relationship to other PRs

PR #6 (security-audit, not yet merged) adds a separate security-audit job with its own weekly schedule. This PR is independent — it adds the schedule to the existing lint-and-test job only.

Test plan

• on.schedule present in workflow
• Tag-gated jobs still only fire on v* tags
• lint-and-test has no if: guard that would exclude schedule runs

🤖 Generated with Claude Code

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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